The present invention relates to a differential type interference prism (referred to as an interference prism, hereinafter) for use in a differential type laser interference length measuring instrument.
In a differential type laser interference length measuring instrument, a laser beam is branched into reference light flux and measuring light flux, which are respectively introduced to a reference mirror by which the reference light flux is returned, and to a measuring mirror by which the measuring light flux is returned, which are located in the same direction, and then an interference fringe detection signal is obtained from the light flux for optical path difference information using both light flux by a heterodyne method or an interference fringe counting method so that the length can be measured. In order to enhance the stability of the detection signal, it is required that the reference light flux and the measuring flux have optical paths which are as close to each other as possible so that the light path difference between two light paths is not badly influenced by expansion and contraction of an optical element caused by a temperature variation, fluctuation of the atmosphere, or an atmospheric pressure variation. That is, it is preferable that the reference light flux and the measuring light flux are always coaxial because they are influenced in common by the factors described above so that the influence on the optical path difference can be cancelled. However, it is difficult to separate the reference light flux from the measuring light flux. Therefore, in general, the reference light flux and the measuring light flux are separated by more than 10 mm. Consequently, in the differential type laser interference length measuring instrument, the reference light flux has a different optical path from the measuring light flux, resulting in considerable influence on the optical paths of both light flux by the aforementioned disturbance factors. This causes a problem of instability (drift, noise) in the detection signal when length measurement resolving power smaller than one nanometer has to be obtained.
FIG. 4 and FIG. 5 show respectively optical path arrangements of conventional differential type laser interference length measuring instruments. In the drawings, L is laser light flux, R is the reference light flux, M is the measuring light flux, I is the optical path difference information light flux in which the reference light flux and the measuring light flux are coaxial, and from which the interference fringe detection signal can be obtained by a conventionally known heterodyne method or an interference fringe counting method, 1 is a polarized light beam splitter, 11 is a polarized light shearing plate, 2 is a .lambda./2 phase shifting plate, 3 is a .lambda./4 phase shifting plate, 4 is a corner cube prism, 5 is a reflection mirror, 6 is a reference mirror, 7 is a measuring mirror, and 74 is a corner cube prism which has the same function as the measuring mirror. As shown in FIG. 4 and FIG. 5, the conventional differential type length measuring instrument has many redundant structures of the optical path using cube prisms in order to obtain an optical path arrangement of the differential type, and therefore, the optical path length of branched reference light flux R and measuring light flux M extends to more than 200 mm, and the optical path length in optical elements becomes more than 80% in an interference prism. Therefore, there is a problem in which the optical path difference between the optical path of reference light flux R and that of measuring light flux M is considerably varied by the influence of disturbances generated transiently in the optical path, such as imbalance of a density or a dimension of the optical element caused by a temperature variation, so that it is difficult to measure a length having an accuracy higher than a nanometer.